I'm debating whether I should upgrade my lens or body. I have a Rebel XTi with a 18-55 kit lens, 50mm 1.4 and a 28 - 135. Do I purchase a new body - likely a 60D or 7D or go for the 15-85? Are there any reasons for not pairing this lens with an old Rebel?

My motivation for purchasing a new camera is higher ISO and faster focusing and my motivation for the purchasing a 15-85 is the nice range and IS.

I just don't want to spend it all at once. It doesn't seem sensible to get a 60D or 7D use it with an old pre-IS 18-55.